converted to fast-twitch fibers with androgen at any point during development, although the treatment time required is considerably less in juveniles
than in adults (Marin et al. 1990). As is the case for muscle fiber number,
masculinization of muscle fiber type by androgen relies on another
hormone, prolactin in this case.Thyroxine secretion during the metamorphic
climax of tadpole life triggers synthesis and secretion of prolactin from the
anterior pituitary. The androgen-sensitive fast-twitch myosin, LM, requires
prolactin and then androgen for its own expression (Edwards et al. 1999).
Thus, the critical period for androgen regulation of muscle fiber type is
opened by the secretion of prolactin from the pituitary but never closes. The
effects of androgen on both cell number and cell types are irreversible: an
adult male that has been castrated for 3 years retains a full complement of
fast-twitch fibers (Watson et al. 1993).That the critical period does not close,
however, is supported by the observation that masculinization can be reinstated in males castrated as juveniles and in females at any stage.
To make the jump from the cellular to the behavioral level requires
knowing whether a particular masculinized cellular property is required for
the exhibition of masculinized vocal behaviors. In this respect, the hormonal
control of cell number appears to be key. Genetic females do not produce
the specific temporal pattern of male advertisement calls unless laryngeal
muscle fiber number has been masculinized (Watson et al. 1993). Even if
every muscle fiber in the larynx is fast-twitch, a female will not call like a
male unless her muscle fiber complement is also masculinized. Why is this
the case? Some clues come from recent studies of how vocal behaviors are
actually generated by male and female brains.
The brains of males and females are functionally differentiated such that
the motor patterns produced by the vocal pathways in the CNS are dramatically different between male and female. To examine the contribution
of CNS to generating vocal patterns, we developed a preparation in which
both laryngeal nerve activity and electromyograms can be recorded from
awake, vocalizing frogs (Yamaguchi and Kelley 2000). Recordings reveal
that the CNS of the sexes produces patterned activity that closely matches
each vocalization while the larynx acts as a faithful transducer of nerve
activity into sound (Fig. 6.8). Thus, the CNS is the source of sexually differentiated vocalizations in Xenopus laevis. Neuronal activity underlying
different male call types is distinct; some calls are likely to be generated by
synchronous firing of motor neuron populations either of constant size or
progressively larger sizes, whereas others are generated by asynchronous
activity of motor neurons, a pattern shared with vocal production in
females. We suggest that these distinct neuronal activity patterns in males
may be subserved by two populations of motor units in males that can be
distinguished by the strength of the neuromuscular synapse.
The robust sex difference in vocal output of the CNS is most likely due
to a sexually differentiated program that changes the generation of patterned output from laryngeal motor neurons. Candidate substrates for the
304
A. Yamaguchi and D.B. Kelley
than in adults (Marin et al. 1990). As is the case for muscle fiber number,
masculinization of muscle fiber type by androgen relies on another
hormone, prolactin in this case.Thyroxine secretion during the metamorphic
climax of tadpole life triggers synthesis and secretion of prolactin from the
anterior pituitary. The androgen-sensitive fast-twitch myosin, LM, requires
prolactin and then androgen for its own expression (Edwards et al. 1999).
Thus, the critical period for androgen regulation of muscle fiber type is
opened by the secretion of prolactin from the pituitary but never closes. The
effects of androgen on both cell number and cell types are irreversible: an
adult male that has been castrated for 3 years retains a full complement of
fast-twitch fibers (Watson et al. 1993).That the critical period does not close,
however, is supported by the observation that masculinization can be reinstated in males castrated as juveniles and in females at any stage.
To make the jump from the cellular to the behavioral level requires
knowing whether a particular masculinized cellular property is required for
the exhibition of masculinized vocal behaviors. In this respect, the hormonal
control of cell number appears to be key. Genetic females do not produce
the specific temporal pattern of male advertisement calls unless laryngeal
muscle fiber number has been masculinized (Watson et al. 1993). Even if
every muscle fiber in the larynx is fast-twitch, a female will not call like a
male unless her muscle fiber complement is also masculinized. Why is this
the case? Some clues come from recent studies of how vocal behaviors are
actually generated by male and female brains.
The brains of males and females are functionally differentiated such that
the motor patterns produced by the vocal pathways in the CNS are dramatically different between male and female. To examine the contribution
of CNS to generating vocal patterns, we developed a preparation in which
both laryngeal nerve activity and electromyograms can be recorded from
awake, vocalizing frogs (Yamaguchi and Kelley 2000). Recordings reveal
that the CNS of the sexes produces patterned activity that closely matches
each vocalization while the larynx acts as a faithful transducer of nerve
activity into sound (Fig. 6.8). Thus, the CNS is the source of sexually differentiated vocalizations in Xenopus laevis. Neuronal activity underlying
different male call types is distinct; some calls are likely to be generated by
synchronous firing of motor neuron populations either of constant size or
progressively larger sizes, whereas others are generated by asynchronous
activity of motor neurons, a pattern shared with vocal production in
females. We suggest that these distinct neuronal activity patterns in males
may be subserved by two populations of motor units in males that can be
distinguished by the strength of the neuromuscular synapse.
The robust sex difference in vocal output of the CNS is most likely due
to a sexually differentiated program that changes the generation of patterned output from laryngeal motor neurons. Candidate substrates for the
304
A. Yamaguchi and D.B. Kelley